The invention relates to a multichannel radio-frequency receiver.
In radio-frequency receivers for radar systems, communication appliances, test equipment etc., the dynamic range is limited by the electronic components that are used, and in the case of digital processing in particular the analogue/digital converters that are used.
In order to increase the dynamic range, it is known from GB 2 204 200 A for a received radio-frequency signal to be matched to the dynamic range of the components that are used by variable amplification after conversion to an intermediate frequency followed by conversion to a baseband frequency, by means of automatic gain control (AGC). However, the variable gain adversely affects the signal quality. In order to make it possible to react to rapid changes in the signal strength, the useful signal must also be delayed with respect to the actuating signal since, otherwise, the automatic gain control cannot carry out the control process before the useful signal arrives at an assembly which limits the dynamic range. This can be achieved only with a great amount of complexity. Finally, the gain setting of the AGC must be known accurately for calibration of measuring radars, in particular meteorological radars.
As an alternative to automatic gain control, EP 0 660 539 B1 proposes that a signal be split into three channels after the radio-frequency signal has been converted to an intermediate frequency. One channel has an amplifier and is supplied to one input of a multiplexer, a further channel is supplied without any change to a further input of the multiplexer, and the last channel has a detector for the signal strength and is connected to a control input of the multiplexer in order to pass on either the amplified channel or the unamplified channel to a common evaluation circuit for the signal, depending on the signal strength. However this requires an additional channel which is not used in the signal evaluation and, furthermore, reduces the signal strength on the other two channels. In addition, the multiplexer corrupts the signal that is passed on to the evaluation circuit, in particular during switching, so that the known receiver is not suitable for signals in which weak amplitudes frequently alternate with strong amplitudes. Finally, the components upstream of the signal divider must be designed for the entire dynamic range of the receiver, and are therefore subject to compromise.